1. Field of the Invention
The present invention relates generally to delay lines and more particularly to a novel optical fiber delay line.
2. Description of Related Art
In various types of signal processing, it is desirable to obtain a true time delay of the signal being processed. For example, radar systems, electronic antenna beam steering systems and clutter rejection filters all require the use of delay lines. In radio frequency (RF) systems, prior art delay lines exemplarily include quartz, surface acoustic wave (SAW), coaxial and microstrip waveguide, and digital delay lines.
For example, a typical SAW delay line has a piezoelectric crystal substrate. On the surface of the substrate are input and output interdigitating fingers. Excitation of the input fingers with an RF signal develops a mechanical wave on the surface of the substrate. The surface wave travels to the output fingers where an output voltage is developed. In the SAW device, the propagation delay is determined by the spacing between the input and output fingers. To achieve useful time delays required in many RF systems, the spacing between the input and output fingers is usually a few orders of magnitude larger than the wavelength of the propagating wave. Since propagating waves decay as an exponential function of the ratio of propagation distance to wavelength, a significant disadvantage of and limitation of the SAW device is its high insertion loss from the decay of the mechanical wave. A typical SAW device insertion loss may typically be in the order of 100 db depending on the desired delay.
Furthermore, the interstitial spacing between the fingers of each of the input and output fingers determines the resonant wavelength of the surface wave and the number of fingers determines the relative efficiency of the electro-mechanical coupling. Since a relatively large number of interspaced fingers are required to minimize the electro-mechanical coupling loss, the relatively large number of fingers makes the SAW device highly frequency selective about the resonant frequency. Therefore, a further disadvantage and limitation of the SAW delay line is the small frequency range over which the device will operate. Obviously, reducing the number of interspaced fingers decreases the frequency selectivity but disadvantageously increase electro-mechanical coupling losses.
Therefore, it is apparent that a SAW delay line can operate only in a narrow frequency band with high insertion loss. Yet another disadvantage and limitation of the SAW delay line is that, after the mechanical wave passes through the output fingers and reaches the edge of the substrate, it is reflected back to the output fingers which may cause echoes in the delayed output signal.
It is also highly desirable to provide a variable delay line. As described hereinabove, the delay of the SAW delay line is determined solely by the fabricated spacing between the input and output fingers on the surface of the substrate. For achieving a variable delay using SAW devices, many such substrates, each substrate having a different length between the input and output fingers, need to be used for each delay line. To fabricate two delay lines on a single substrate would cause mechanical waves to couple and interfere with each other.
Digital delay lines overcome some of the limitations and disadvantages of the saw and quartz delay lines, such as insertion and coupling losses and variability of delay, and are particularly useful when very long delays of an RF signal are needed. A digital delay line mixes the RF signal with a lower frequency signal to obtain an in-phase and quadrature data. The data is then mixed back up to the original signal frequency. A significant disadvantage and limitation of the digital delay is that it requires extensive hardware, and the mixing of signals may introduce other spurious signals. A further disadvantage and limitation of the digital delay line is that, since it operates at radio frequencies, it is highly susceptible to electro-magnetic interference (EMI). Also, stray propagation along ground loops may cause cross-talk in the digital delay line.